This invention relates to a current transformer for measuring an electric current flowing through three phase bus bar conductors.
FIG. 9 is a schematic illustration of a conventional current transformer disclosed in Japanese Patent No. 2600548 and FIG. 10 is a cross-sectional view taken along line A--A of the current transformer of FIG. 9. In the figures, reference numeral 1 is one of the three-phase bus bar conductors which is a line to be measured in terms of the electric current. 2a and 2b are iron cores for defining a magnetic path intersecting the bust conductor 1, and 3a and 3b are secondary windings wound on the iron cores 2a and 2b for measuring the electric current flowing through the bus conductor 1 to be measured. The secondary windings 3a and 3b are not illustrated in FIG. 9.
4a and 4b are shield windings, each of which comprises four coils having the equal number of windings wound on the iron cores 2a and 2b to extend over the equal circumferential distance of the iron core. The shield windings 4a and 4b respectively surround in intimate contact the current transformers 8a and 8b which are disposed in the direction of the longitudinal axis of the bus conductor 1. The shield windings 4a and 4b are for alleviating the influence of the electric current flowing through the neighboring bus conductors 6a and 6b on the electric current flowing through the secondary windings of the bus conductor 1. 5 is a connection line for connecting together the terminals of the same polarity of the shield winding 4.
6a are two of three-phase bus conductors, which are adjacent to the bus conductor 1 to be measured. 6b are conductors connecting the bus conductor 1 and the bus conductors 6a to each other for providing a neutral point, which is a junction between the bus conductor 1 and the bus conductors 6b. 8a and 8b are current transformers (8a and 8b are transformer units, which generally referred to as a transformer), which are composed of iron cores 2, 2a and 2b, secondary windings 3a and 3b and shield windings 4, 4a and 4b, respectively. L is a distance between the iron cores 2a and 2b.
The operation of this current transformer will now be described. In FIGS. 9 and 10, the electric currents flowing through the secondary windings 3a and 3b are proportional to the current flowing through the bus conductor 1 to be measured, so that the electric current flowing through the bus conductor 1 can be measured by the secondary windings 3a and 3b. The shield windings 4a and 4b are wound around the iron cores 2, 2a and 2b, respectively, divided into four along the circumference of the iron core and the same polarity of each coil is connected together by the connection line 5, so that the electric current flowing through the bus conductors 6a and 6b induces an electric current in the shield windings 4a and 4b, thereby reducing the magnetic flux penetrating into the iron cores 2, 2a and 2b. Also, it is possible to ensure that the magnetic flux generated by the induced current does not affect the current in the bus conductor 1 to be measured.
However, since there are two secondary windings and two shield windings between the iron cores 2a and 2b, generating a large amount of heat and since the heat dissipating surface area at this portion is small, the current transformer generates a large amount of heat.
Also, as for the current transformer 8a far from the bus conductor 6b constituting the neutral point, the mutual inductance between the bus conductor 6b and the shield winding 4a is small and the electric current induced in the shield current is small. On the other hand, as for the current transformer 8b close to the bus conductor 8a, the mutual inductance between the bus conductor 6b and the shield winding 4b is large and the current induced within the shield windings is large generating a large heat at the current transformer 8b close to the bus conductor 6b. Therefore, a problem has been posed that a material having a good heat resistivity and a winding having a large winding diameter must be used.
Also, when the distance L between the iron cores 2a and 2b is large, the magnetic flux from the bus conductors 6a can easily penetrate, so that a large current is induced in the shield windings 4a and 4b, generating a large heat in the transformer and the magnetic flux is apt to concentrate at the iron cores 2, 2a and 2b, posing a problem that the cross-sectional area of the iron core must be made large for achieving the precise current measurement of the bus conductor 1.
Further, the shield windings 4a and 4b are divided into the coil sections of an even number disposed around the transformer 8 and have the same polarity connected together, but since the mutual inductance between the respective divided coil sections and the bus conductors 6a and 6b are different, making the induced current imbalance, generating a local high temperature in the current transformer.